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There are many well-established charts that have been in use for a long time; however, marked differences between populations sometimes force researchers to build new nomograms for the different races.

This parameter is useful in the early stages of pregnancy when the fetal pole or the embryo is still not identifiable on ultrasound.

Bethesda, MD: National Council on Radiation Protection and Measurements; 2002. Effective prenatal diagnosis relies on a high standard of imaging.

Several national and international bodies have described standards for imaging in the first, second, and third trimester of pregnancy.

Three-dimensional (3D) ultrasound can be a useful adjunct to 2D ultrasound in select circumstances and will be discussed in Chapter 2.

All stated gestational ages are according to last menstrual period dating.

This chapter deals with normal fetal anatomy; however, frequent references to anomalies are made to underscore the pertinence of a good anatomic evaluation.

Figure 12–3 Scan demonstrating the measurement of the head circumference.■ There is no epidemiologic support for a causal relationship between diagnostic ultrasound during pregnancy and adverse biologic effects to the fetus observed for outputs under a spatial-peak temporal-average intensity of 94 m W/cm From Fowlkes JB; Bioeffects Committee of the American Institute of Ultrasound in Medicine. National Council on Radiation Protection and Measurements.American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound. Exposure Criteria for Medical Ultrasound, II: Criteria Based on All Known Mechanisms.According to one study by Muller et al, three-dimensional volumetric scans are more accurate than two-dimensional scans in obtaining the gestational sac volume, but have shown to be of no prognostic significance for gestational outcome. One must be careful not to include the yolk sac in the measurement (Fig. The CRL can be used to assess the gestational age between 6 and 14 weeks (Table 12–2).The mean gestational age was calculated from a regression equation. Its best accuracy is from 6 to 10 weeks, during which time it has an error of ± 3 to 4 days (95% confidence interval).

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